The principles and interpretation of annular dark-field Z-contrast imaging

Annular dark-field imaging in a scanning transmission electron microscope is the implementation in TEM of incoherent imaging. The complicated interference fringes seen in coherent imaging do contain information, but it is not easily invertible to a specimen structure. Generally the only way to proceed is to simulate images from trial structures to match against the images. The removal of information in the incoherent image formation process simplifies the interpretation of the data, and enough information is retained to determine the object structure. This allows the opportunity to observe unexpected structures, which is of crucial importance. The type of information that can be derived from an ADF image falls into two broad classes, the determination of the projected position of atoms and atomic columns, and the determination of the elemental composition of the specimen. The phase randomization by the phonon scattering can help in destroying the coherence between the scattering from different atoms, but the coherence destruction process is much less complete in this longitudinal direction.